1. Field of the Invention
This invention relates to a clamping circuit, and more particularly to a clamping circuit for preventing occurrence of the deep saturation state of an output driving transistor provided in an output driving circuit.
2. Description of the Related Art
Conventionally, a Schottky barrier diode SBD having an anode connected to the base B of an output driving transistor Q1 shown in FIG. 1 and a cathode connected to the collector C thereof is used as a clamping circuit for preventing occurrence of the deep saturation state of the output driving transistor.
The clamping circuit is activated when the collector voltage or the output voltage (output signal) V.sub.OUT is set to satisfy the following expression. EQU V.sub.OUT &lt;V.sub.BE -V.sub.FD1
where V.sub.OUT is the output voltage, V.sub.BE is a voltage between the base and emitter of a transistor Q100 and V.sub.FD1 is the threshold voltage of the diode SBD.
When the clamping circuit is activated, the base current i.sub.B of the transistor Q100 decreases and part of the current i.sub.B which has flowed into the base B is caused to flow into the collector C via the SBD. As a result, the transistor Q100 can be prevented from being set into the deep saturation state.
Unless the clamping circuit is used, the base current rent i.sub.B is kept unchanged even when the output voltage V.sub.OUT becomes lower, and the base-emitter junction and the base-collector junction are biased in a forward direction. Even if the base current i.sub.B is interrupted in this state, it takes a long time for minority carriers accumulated in the base B to disappear. As a result, the transition time of the output from the "L" level to the "H" level becomes long.
However, since the SBD is used in the above clamping circuit, there occurs a problem that the manufacturing process becomes complicated, for example. Particularly, in the case of a gate array using the master slice method, it is necessary to form the SBD of the clamping circuit on the master slice in order to constitute the clamping circuit having the SBD. However, the area for the SBD becomes useless in an area of transistors (basic cells) which are not used for constituting the output driver.
In order to solve the above problem, it is considered to use a circuit including a silicon bipolar transistor Q102 having the base B and the collector C connected together in the clamping circuit a shown in FIG. 2.
In this case, since V.sub.BE of the output driving transistor Q100 and the threshold voltage V.sub.FD2 of the base-emitter junction of the transistor Q102 are equal to each other, a resistor R2 is connected at one end to the base B of the transistor Q100 and connected at the other end to the base B and the collector C of the transistor Q102.
This type of clamping circuit is activated when the collector voltage or the output voltage (output signal) V.sub.OUT satisfies the following expression. EQU V.sub.OUT &lt;V.sub.BE +i.sub.B .times.R2-V.sub.FD2 =i.sub.B .times.R2
where V.sub.OUT is the output voltage, V.sub.BE is a voltage between the base and emitter of the transistor Q100, i.sub.B is the base current, R2 is the resistance of the resistor and V.sub.FD2 is the threshold voltage of the base emitter junction of the transistor Q102.
The clamping circuit is activated after the V.sub.OUT becomes lower than i.sub.B R2. Therefore, the base current i.sub.B can be decreased.
However, when the base current i.sub.B is decreased, the voltage drop across the resistor R2 becomes smaller to reduce the current flowing in the base-emitter junction of the transistor Q102. As a result, it becomes difficult to abruptly increase the current flowing in the base-emitter junction of the transistor Q102 or reduce the base current i.sub.B after the output voltage V.sub.OUT has reached a preset level.
When the SBD is used, the voltage-current characteristic of the clamping circuit can be so determined that a current can be caused to more easily flow in comparison with a case of use of the PN junction by adequately selecting metal material constituting one of the electrodes of the SBD. However, when the base-emitter junction of the silicon transistor is used, current does not easily flow into the PN junction and reduction in the base current i.sub.B becomes dull in comparison with a case of using an SBD.
This problem may be solved by increasing the resistance of the resistor R2 and activating the clamping circuit when the output voltage V.sub.OUT is sufficiently large. However, at this time, it becomes difficult to realize the DC characteristic required for the output driver circuit.